This invention relates to systems for scanning a surface.
Known scanning systems include laser printers for "writing" an image on a two-dimensional surface and facsimile image transmission devices that "read" an image from such a surface.
In a typical scanning system, the image surface (medium) is scanned in a raster fashion by a fast moving mechanically rotated beam deflector (such as a mirror) in the fast scan direction, X. Scanning in the slow scan direction, Y, is accomplished either by a slow moving beam deflector (driven, e.g., by a galvanometer), or by moving either the medium or the scanning system relative to the other. In all types of systems a lens assembly is used for maintaining focus as the scanning proceeds.
In laser scanning systems, the scanning elements occupy either a pre-objective or post-objective position. In a pre-objective system, the beam from the laser passes through the focusing lens after being deflected. In a post-objective system, the lens is ahead of the scanning elements.
Although a simple lens can be used for a post-objective scanning system, such systems ordinarily produce a curved focal surface and thus require either a curved medium or a scan angle which is small enough that the curved focal surface falls within the beam's depth of focus.
A pre-objective system can produce a flat focal surface, but this requires a complex (e.g. flat field or F-theta) lens.
By providing a post-objective scanning system with two lenses, focus can be achieved anywhere on the medium by varying the spacing between the lenses. If the scanning speed is slow, one or both lenses can be moved by a linear actuator to track the changing distance to the medium during scanning. However, as the scanning speed is increased, the frequency response of a linear actuator will eventually be exceeded.